Tunable laser with integrated wavelength reference

ABSTRACT

In the prior art, tunable lasers utilizing silicon-based tunable ring filters and III-V semiconductor-based gain regions required the heterogeneous integration of independently formed silicon and III-V semiconductor based optical elements, resulting in large optical devices requiring a complex manufacturing process (e.g., airtight packaging to couple the devices formed on different substrates, precise alignment for the elements, etc.). Embodiments of the invention eliminate the need for bulk optical elements and hermetic packaging, via the use of hybridized III-V/silicon gain regions and silicon optical components, such as silicon wavelength filters and silicon wavelength references, thereby reducing the size and manufacturing complexity of tunable lasing devices. For example, embodiments of the invention may utilize hybridized III-V/silicon gain regions with ring filters on silicon form a tunable laser with efficient gain from the III-V region, while providing wide tunability, efficient tunability, and narrow linewidth due to the nature of the silicon rings.

TECHNICAL FIELD

This disclosure relates generally to the field of photonics, and inparticular but not exclusively, tunable semiconductor lasers.

BACKGROUND

Wavelength tunable semiconductor laser devices supply optical power andsignals used in applications where precise wavelength control must bemaintained, such as optical wavelength division multiplexing (WDM)communication systems and fiber optic networks. The wavelength of theoutput of tunable laser devices varies based on a feedback controlsystem. This control system will utilize some form of wavelengthdependent optical filters to control the wavelength of the light inputto the gain medium of the laser device. Wavelength references willfurther stabilize the laser output of the device.

Existing tunable lasers may use a combination of independentlyfabricated silicon and III-V semiconductor based optical components.III-V semiconductor materials are not as effective compared to siliconmaterials for making low loss, compact, wavelength tunable filters,while silicon materials are not as optically efficient as III-Vsemiconductor materials for emitting light. Thus, in the prior art,utilizing silicon-based wavelength filters and having optically activeregions with the optical efficiency of III-V semiconductor-based devicesrequires the combination of separately formed silicon and III-Vsemiconductor based optical elements. This combination of separatelyformed optical elements results in a larger optical devices requiring acomplex manufacturing processes not on a wafer scale (e.g., hermeticpackaging to couple the devices formed on different substrates, precisealignment for the elements, etc.).

DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified. It should be appreciated that the followingfigures may not be drawn to scale.

FIG. 1 is a block diagram of a tunable laser according to an embodimentof the invention.

FIG. 2 is a block diagram of a tunable laser having a thermal basedwavelength reference according to an embodiment of the invention

FIG. 3 is a block diagram of a tunable laser having a ring topographyaccording to an embodiment of the invention.

FIG. 4 is a block diagram of a tunable laser outputting lasing light viaa drop port of a tunable ring filter according to an embodiment of theinvention.

FIG. 5 is a diagram of an optical system utilizing a plurality ofcascaded tunable ring filters according to an embodiment of theinvention.

Descriptions of certain details and implementations follow, including adescription of the figures, which may depict some or all of theembodiments described below, as well as discussing other potentialembodiments or implementations of the inventive concepts presentedherein. An overview of embodiments of the invention is provided below,followed by a more detailed description with reference to the drawings.

SPECIFICATION

Embodiments of an apparatus, system and method to utilize a siliconsubstrate-based tunable laser having a wavelength dependent opticalfilter and a wavelength reference formed in the silicon substrate aredescribed herein. Said silicon-based tunable laser will further have again region at least partially formed in the silicon substrate. In thefollowing description numerous specific details are set forth to providea thorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a block diagram of a tunable laser according to an embodimentof the invention. In this embodiment, laser 100 includes tunable ringfilter (TRF) 110, gain region 120, phase control region 130, outputcoupler 140 and stabilized wavelength reference 150. The abovecomponents may be formed in passive regions of silicon substrate 190,except gain region 120, which may be partially formed in an activeregion of silicon substrate 190 (in one embodiment described below, gainregion 120 further includes III-V semiconductor material).

TRF 110 selects the wavelength for laser 100. TRF 110 includes a tuningregion which, when a bias voltage is applied, determines the specificwavelength of the signal received by gain region 120. Phase controlregion 130 may perform fine control of the laser characteristics oflight received from gain region 120. For example, phase control region130 may adjust the resonant cavity modes associated with the tunablelaser. Output coupler 140 extracts at least a portion of the laser lightfrom phase control region 130, and outputs the laser light. Outputcoupler 140 may further direct a portion of the light to stabilizedwavelength reference 150.

In the illustrated embodiment, wavelength reference 150 is used toprovide wavelength stability for laser 100. Wavelength reference 150receives a portion of the laser output for output coupler 140 to provideeasy and accurate wavelength determination of the laser output.Wavelength reference 150 may comprise an etalon, interference filter,grating or any functionally equivalent means that passes light at aknown wavelength.

Wavelength reference 150 may be utilized to determine a differencebetween the light from the gain region to a known wavelength value. TRF110 may then be adjusted based, at least in part, on this determineddifference in order to tune the output of laser 100 (e.g., via anelectrical signal sent from wavelength reference 150).

In one embodiment, gain region 120 comprises a first region of formed insilicon material of substrate 190 and a second region of non-siliconmaterial with high electro-optic efficiency, such as III-Vsemiconductors. Said regions may be fabricated independently andsubsequently bonded via any bonding process known in the art. Thenon-silicon semiconductor region of gain region 120 may at leastpartially overlap the silicon region to create a lateral overlap region;an optical waveguide of the gain region is included in this lateraloverlap region, and the waveguide includes both the silicon and thenon-silicon material. Thus, the refractive index of at least one of thesilicon material and the non-silicon material within the opticalwaveguide may change based on an electrical difference applied to thegain region.

In one embodiment, the above described non-silicon material is a groupIII-V semiconductor material. III-V semiconductors have elements thatare found in group III and group V of the periodic table (e.g., IndiumPhosphide (InP), Indium Gallium Arsenide Phosphide (InGaAsP), GalliumIndium Arsenide Nitride (GaInAsN)). The carrier dispersion effects ofIII-V based materials may be significantly higher than in silicon basedmaterials for bandgaps closer to the wavelength of the light traversingthrough gain region 120, as electron speed in III-V semiconductors ismuch faster than that in silicon. Thus, III-V semiconductor materialsenable photonic operation with an increased efficiency at generatinglight from electricity and converting light back into electricity.

Thus, the active waveguide of gain region 120 is at least partiallyformed in (or on) the silicon material of substrate 190, and TRF 110,phase control 130 and wavelength reference 150 are also formed in (oron) the silicon material of the substrate. For example, wavelengthreference 150 may be formed in or on the silicon material of thesubstrate by depositing resistive material in or on the silicon, dopingthe silicon material, or any other functionally equivalent means. Thetuning efficiency of silicon-based TRFs is higher than III-Vsemiconductor-based TRFs, and silicon-based TRFs have narrowerlinewidths.

III-V semiconductor materials are not as efficient compared to siliconfor making low loss, efficient TRFs, while silicon semiconductormaterials cannot produce an efficient optical gain without heterogeneousintegration of III-V semiconductor material. Thus, in the prior art,utilizing silicon-based TRFs and having the optical efficiency of III-Vsemiconductor-based gain regions required packaging of silicon and III-Vsemiconductor based optical elements, resulting in large optical devicesthat require a complex manufacturing process. Therefore, embodiments ofthe invention such as the embodiment illustrated in FIG. 1 eliminate theneed for bulk optical elements and hermetic packaging, thereby reducingthe size and manufacturing complexity of tunable lasing devices.

FIG. 2 is a block diagram of a tunable laser having a thermal basedwavelength reference according to an embodiment of the invention. Inthis embodiment, laser 200 includes TRF 210, gain region 220, phasecontrol region 230 and output coupler 240. As described above, each ofthe described elements are either partially or entirely formed in thesilicon material of substrate 290, enabling tunable laser 200 to becontrolled to the proper wavelength without integrating additionaloptical components—i.e., optical components not formed from the siliconmaterial of the substrate. TRF 210 may include a heating elementassociated with an optical ring resonator. Said heating element may beadjusted, based on optical feedback received from output coupler 240, inorder to alter the wavelength of laser light passed to gain region 220;said heating element may also be sized to ensure substantial transfer ofheat to the respective optical waveguide, and to limit absorbance by theheating element of optical radiation propagating through the respectiveoptical waveguide.

In this embodiment, TRF 210 includes wave stabilization functionality.In one embodiment, highly stable thermal sensors are integrated toprovide a stable wavelength reference for the wavelength of the laserlight passed from TRF 210 to gain region 220. Thus, in this embodimentof the invention, the wavelength reference is to be integrated withinthe laser cavity. Highly stable resistive temperature devices (RTDs)formed from silicon substrate 290 may be used to measure the temperatureof the wavelength filtering elements. The cavity filter is arranged suchthat changes in the refractive index due to stress are minimized and thetemperature is directly related to the laser wavelength and thus,determining the laser wavelength may be done by measuring thetemperature of the wavelength filtering elements via the above describedRTDs.

By eliminating the need to integrate additional optical components fortunable lasers, embodiments of the invention may form other variants oftunable lasers with a reduced device footprint. FIG. 3 is a blockdiagram of a tunable laser having an alternative topography according toan embodiment of the invention. In this embodiment, laser 300 includesTRF 310, gain 320 and output coupler 330 configured in a ring topographywhich could produce lower linewidth lasers due to the unidirectionallight propogation in the laser (as opposed to the bi-directional lightpropagation illustrated in FIG. 2). As described above, each of thesedescribed elements are either partially or entirely formed in thesilicon material of substrate 390.

FIG. 4 is a block diagram of a tunable laser outputting lasing light viaa drop port of a tunable ring filter according to an embodiment of theinvention. In this embodiment, laser 400 includes TRF 410 and gainregion 420. As described above, each of these described elements areeither partially or entirely formed in the silicon material of substrate490.

The use of an output coupler, as described in previous exampleembodiments, may be eliminated. For laser 400, light is output via dropport 415 of TRF 410, further reducing the device footprint necessary fora wavelength tunable laser.

FIG. 5 is a diagram of an optical system utilizing a plurality ofcascaded tunable ring filters according to an embodiment of theinvention. In this embodiment, optical system 500 includes tunable laser501 and silicon evanescent electro-absorption modulator 502. The tunablelaser further includes tunable ring feedback coupler 510, siliconevanescent gain 520 and laser output coupler 530.

Tuning ring feedback coupler 510 may include cascaded ring filters 515.In this embodiment, the ring filters each have slightly different radiiand are laterally coupled to input/output waveguide 540 in a cascadedarrangement. It is understood that varying the radii of ring filters 515slightly allows for a wide range of tuning for laser 501, due to theVernier effect (i.e., a large tuning effect is accomplished byexploiting the Vernier effect, by which small relative refractive indexchanges may be used to yield large relative wavelength changes).

Thus, the use of a hybridized III-V/silicon gain region (hybrid siliconlaser or hybrid ridge lasers) with cascaded Vernier ring filters formedfrom the silicon material of substrate 590 allow for efficient gain fromthe III-V region, while providing wide tunability, efficient tunability,and narrow linewidth due to the narrow linewidth nature of siliconrings.

Modulator 502 may perform either amplitude or phase modulation of thelight received from tunable laser 501. Modulator 502 may include awaveguide comprising silicon and III-V semiconductor material, similarto the gain regions described above. In one embodiment, optical system500 is included in a single device or chip, wherein silicon componentsof system 500 are included on a silicon portion of the chip, and III-Vsemiconductor components of system 500 are included on a III-V portionof the chip. Said portions may be fabricated independently andsubsequently bonded via any bonding process known in the art.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

The invention claimed is:
 1. A tunable laser, comprising: a siliconsubstrate defining a waveguide, the waveguide being configured topropagate light, the silicon substrate including a gain regionconfigured to amplify light propagating through the waveguide andincluding a silicon material and a III-V semiconductor material, thesilicon substrate further including a passive region configured tofilter a wavelength of light propagating through the waveguide, thepassive region including at least a portion of a laser cavity, the lasercavity arranged in a ring topography so that light propagates in asingle direction within the laser cavity; an output coupler formed in apassive region of the silicon substrate, the output coupler configuredto allow amplified and filtered light to exit the waveguide, the outputcoupler further configured to produce optical feedback from theamplified and filtered light in the waveguide; and a heating elementdisposed proximate the waveguide, the heating element configured toadjust a temperature of the passive region in response to the opticalfeedback thereby to adjust the wavelength of light filtered in thewaveguide.
 2. The tunable laser of claim 1, wherein the passive regionof the silicon substrate includes a tunable ring filter.
 3. The tunablelaser of claim 1, wherein the passive region of the silicon substrateincludes a plurality of cascaded tunable ring filters, each tunable ringfilter having a different radius value.
 4. The tunable laser of claim 3,wherein the waveguide extends through an electroabsorption modulatorconfigured to perform amplitude or phase modulation of light propagatingwithin the waveguide.
 5. The tunable laser of claim 4, wherein theoutput coupler is positioned along the waveguide between theelectroabsorption modulator and the plurality of cascaded tunable ringfilters.
 6. The tunable laser of claim 1, wherein the waveguide extendsthrough a phase control region formed in a passive region of the siliconsubstrate, the phase control region being configured to adjust resonantcavity modes of the laser cavity.
 7. The tunable laser of claim 6,wherein the phase control region is positioned along the waveguidebetween the wavelength reference and the gain region.